Conventionally, a variety of separating membranes have been proposed for removing corpuscles from blood to obtain plasma or serum required for laboratory tests.
For example, Japanese Examined Patent Publication No. 2-23831 (1990) discloses a method of colleting plasma from blood using a hollow fiber having fine pores of 0.05 to 1 μm in diameter, a porosity of outer surface of not more than 40%, and a porosity of inner surface of not less than 60%.
Japanese Examined Patent publication No. 6-64054 (1994) proposes a method of separating plasma or serum through a fiber layer having a mean diameter of 0.2 to 5.0 μm and a density of 0.1 to 0.5 g/cm3.
On the other hand, Japanese Unexamined Patent Publication No. 11-285607 (1999) discloses a separation method which utilizes difference in movement speed between corpuscles and plasma or serum components through a polymeric microfiber assembly or a porous polymer. In this method, a hydrophilic polymer is immobilized on a surface of fiber, the hydrophilic polymer swells after separation of plasma or serum, the filter is clogged, and thereby filtration is automatically stopped.
However, the method disclosed in Japanese Examined Patent Publication No. 2-23831 (1990) had an economic problem because high cost is required to produce a disposable product due to the hollow fiber.
The method of Japanese Examined Patent Publication No. 6-64054 enables separation of plasma or serum, however the filtering speed is still unsatisfactory. Application of pressure to improve the filtering speed could sometimes cause breakage of corpuscles, hemolysis, and contamination of separated plasma or serum with leaking erythrocytes. Additionally, in the blood in which fibrin or the like precipitates, hemolysis was more likely to occur since clogging was more likely to occur during the separation process.
In the method of Japanese Unexamined Patent Publication JP-A 11-285607 (1999), since the filtering speed changes between bloods of different hematocrits and viscosities, it was impossible to securely stop the filtration at the point when filtration of plasma or serum completed.